Polymorphic forms alpha, beta and gamma of rifaximin

ABSTRACT

Crystalline polymorphous forms of rifaximin (INN) antibiotic named rifaximin α and rifaximin β, and a poorly crystalline form named rifaximin γ, useful in the production of medicinal preparations containing rifaximin for oral and topical use and obtained by means of a crystallization carried out by hot-dissolving the raw rifaximin in ethyl alcohol and by causing the crystallization of the product by addition of water at a determinate temperature and for a determinate period of time, followed by a drying carried out under controlled conditions until reaching a settled water content in the end product, are the object of the invention.

APPLICATION PRIORITY DATA

This application is a continuation of U.S. application Ser. No.12/119,600, filed May 13, 2008, now U.S. Pat. No. 7,902,206, issued onMar. 8, 2011, which in turn is a continuation-in-part of applicationU.S. application Ser. No. 11/873,841, filed on Oct. 17, 2007, which is acontinuation-in part of U.S. application Ser. No. 11/135,651, filed onMay 24, 2005, now abandoned, which is a continuation-in-part ofPCT/EP04/12490, filed on Nov. 4, 2004, which claims priority to Italianapplication No. MI2003A002144 filed Nov. 7, 2003, all of which areincorporated by reference herein in their entirety, including anydrawings.

BACKGROUND AND SUMMARY OF THE INVENTION

Rifaximin (INN; see The Merck Index, XIII Ed., 8304) is an antibioticbelonging to the rifamycin class, exactly it is a pyrido-imidazorifamycin described and claimed in Italian Patent IT 1154655, whileEuropean Patent EP 0161534 describes and claims a process for itsproduction starting from rifamycin O (The Merck Index, XIII Ed., 8301).

Both these patents describe the purification of rifaximin in a genericway stating that crystallization can be carried out in suitable solventsor solvent systems and summarily showing in some examples that thereaction product can be crystallized from the 7:3 mixture of ethylalcohol/water and can be dried both under atmospheric pressure and undervacuum without specifying in any way either the experimental conditionsof crystallization and drying, or any distinctive crystallographiccharacteristic of the obtained product.

The presence of different polymorphs had just not been noticed andtherefore the experimental conditions described in both patents had beendeveloped with the goal to get a homogeneous product having a suitablepurity from the chemical point of view, independent from thecrystallographic aspects of the product itself.

It has now been found, unexpectedly, that there are several polymorphousforms whose formation, besides the solvent, depends on time andtemperature conditions under which both crystallization and drying arecarried out.

In the present application, these orderly polymorphous forms will be,later on, conventionally identified as rifaximin α (FIG. 1) andrifaximin β (FIG. 2) on the basis of their respective specificdiffractograrns, while the poorly crystalline form with a high contentof amorphous component will be identified as rifaximin γ (FIG. 3).

Rifaximin polymorphous forms have been characterized through thetechnique of the powder X-ray diffraction.

The identification and characterization of these polymorphous forms and,simultaneously, the definition of the experimental conditions forobtaining them is very important for a compound endowed withpharmacological activity which, like rifaximin, is marketed as medicinalpreparation, both for human and veterinary use. In fact it is known thatthe polymorphism of a compound that can be used as active ingredientcontained in a medicinal preparation can influence thepharmaco-toxicologic properties of the drug. Different polymorphousforms of an active ingredient administered as drug under oral or topicalform can modify many properties thereof like bioavailability,solubility, stability, colour, compressibility, flowability andworkability with consequent modification of the profiles oftoxicological safety, clinical effectiveness and productive efficiency.

What mentioned above is confirmed by the fact that the authorities thatregulate the grant of marketing authorization of the drugs marketrequire that the manufacturing methods of the active ingredients arestandardized and controlled in such a way that they give homogeneous andsound results in terms of polymorphism of production batches(CPMP/QWP/96, 2003—Note for Guidance on Chemistry of new ActiveSubstance; CPMP/ICH/367/96—Note for guidance specifications: testprocedures and acceptance criteria for new drug substances and new drugproducts: chemical substances; Date for coming into operation: May2000).

The need for the above-mentioned standardization has further beenstrengthened in the field of the rifamycin antibiotics by Henwood S. Q.,de Villiers M. M., Liebenberg W. and Lötter A. P., Drug Development andIndustrial Pharmacy, 26 (4), 403-408, (2000), who have ascertained thatdifferent production batches of the rifampicin (INN) made from differentmanufacturers differ from each other in that they show differentpolymorphous characteristics, and as a consequence they show differentdissolution profiles, along with a consequent alteration of therespective pharmacological properties.

By applying the crystallization and drying processes genericallydisclosed in the previous patents IT 1154655 and EP 0161534 it has beenfound that under some experimental conditions a poorly crystalline formof rifaximin is obtained, while under other experimental conditionsother polymorphic crystalline forms of Rifaximin are obtained. Moreoverit has been found that some parameters, absolutely not disclosed in theabove-mentioned patents, like for instance preservation conditions andthe relative ambient humidity, have the surprising effect to determinethe polymorph form.

The polymorphous forms of rifaximin object of the present patentapplication were never seen or hypothesized, while thinking that,whichever method was used within the range of the described condition, asole homogeneous product would always have been obtained, irrespectiveof crystallizing, drying and preserving conditions.

It has now been found that the formation of α, β and γ forms dependsboth on the presence of water within the crystallization solvent, on thetemperature at which the product is crystallized and on the amount ofwater present in the product at the end of the drying phase.

Form α, form β and form γ of rifaximin have then been synthesized andthey are the object of the invention.

Moreover it has been found that the presence of water in rifaximin inthe solid state is reversible, so that water absorption and/or releasecan take place in time in presence of suitable ambient conditions;consequently rifaximin is susceptible of transition from one form toanother, also remaining in the solid state, without need to be againdissolved and crystallized. For instance polymorph α, getting water byhydration up to a content higher than 4.5%, turns into polymorph β,which in its turn, losing water by drying up to a content lower than4.5%, turns into polymorph α.

These results have a remarkable importance as they determine theconditions of industrial manufacturing of some steps of working whichcould not be considered critical for the determination of thepolymorphism of a product, like for instance the washing of acrystallized product, or the preservation conditions of the end product,or the characteristics of the container in which the product ispreserved.

The above-mentioned α, β and γ forms can be advantageously used as pureand homogeneous products in the manufacture of medicinal preparationscontaining rifaximin.

As already said, the process for manufacturing rifaximin from rifamycinO disclosed and claimed in EP 0161534 is deficient from the point ofview of the purification and identification of the product obtained; itshows some limits also from the synthetic point of view as regards, forinstance, the very long reaction times, from 16 to 72 hours, not verysuitable to an industrial use and moreover because it does not providefor the in situ reduction of rifaximin oxidized that may be formedwithin the reaction mixture.

Therefore, a further object of the present invention is an improvedprocess for the industrial manufacturing of the α, β and γ forms ofrifaximin, herein claimed as products and usable as defined andhomogeneous active ingredients in the manufacture of the medicinalpreparations containing such active ingredient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a powder X-ray diffractogram of rifaximin polymorphic form α.

FIG. 2 is a powder X-ray diffractogram of rifaximin polymorphic form β.

FIG. 3 is a powder X-ray diffractogram of rifaximin polymorphic form γ.

DETAILED DESCRIPTION OF THE INVENTION

As already said, form α, form β and form γ of the antibiotic known asrifaximin (INN), processes for their production and the use thereof inthe manufacture of medicinal preparations for oral or topical route, areobject of the present invention.

A process object of the present invention comprises reacting one molarequivalent of rifamycin O with an excess of 2-amino-4-methylpyridine,preferably from 2.0 to 3.5 molar equivalents, in a solvent mixtureconsisting of water and ethyl alcohol in volumetric ratios between 1:1and 2:1, for a period of time between 2 and 8 hours at a temperaturebetween 40° C. and 60° C.

When the reaction is over, the reaction mass is cooled at roomtemperature and is added with a solution of ascorbic acid in a mixtureof water, ethyl alcohol and aqueous concentrated hydrochloric acid,under strong stirring, in order to reduce the small amount of oxidizedrifaximin that forms during the reaction and finally the pH is adjustedto about 2.0 by a further addition of hydrochloric acid concentratedaqueous solution, in order to better remove the excess of2-amino-4-methylpyridine used in the reaction. The suspension isfiltered and the obtained solid is washed with the same water/ethylalcohol solvent mixture used in the reaction. Such semi finished productis called “raw rifaximin”.

Raw rifaximin can be directly submitted to the subsequent purificationstep. Alternatively, in case long times of preservation of the semifinished product are expected, raw rifaximin can be dried under vacuumat a temperature lower than 65° C. for a period of time between 6 and 24hours; such semi finished product is called “dried raw rifaximin”.

The so obtained raw rifaximin and/or dried raw rifaximin are purified bydissolution in ethyl alcohol at a temperature between 45° C. and 65° C.and by crystallization by addition of water, preferably in weightamounts between 15% and 70% in respect of the amount by weight of theethyl alcohol used for the dissolution, and keeping the obtainedsuspension at a temperature between 50° C. and 0° C. under stirringduring a period of time between 4 and 36 hours.

The suspension is filtered and the obtained solid is washed with waterand dried under vacuum or under normal pressure, with or without adrying agent, at a temperature between room temperature and 105° C. fora period of time between 2 and 72 hours.

The achievement of α, β and γ forms depends on the conditions chosen forcrystallization. In particular, the composition of the solvent mixturefrom which crystallization is carried out, the temperature at which thereaction mixture is kept after crystallization and the period of time atwhich that temperature is kept, have proven to be critical.

More precisely, rifaximin γ is obtained when the solution is taken to atemperature between 28° C. and 32° C. in order to start precipitationand the obtained suspension is further cooled to 0° C. and kept at thistemperature for a period of time between 6 and 24 hours.

The suspension is filtered, the solid is washed with demineralized waterand dried to a water content between 1.0% and 2.0%.

α and β rifaximins are obtained when the temperature is first taken to avalue between 28° C. and 32° C. to start crystallization, then thesuspension is taken to a temperature between 40° C. and 50° C. and keptat this value for a period of time between 6 and 24 hours, then thesuspension is quickly cooled to 0° C., in a period of time between 15minutes and one hour, is filtered, the solid is washed with water andthen is dried.

The drying step plays an important role in obtaining the α and βpolymorphous forms of rifaximin and must be checked with a suitablemethod fit for the water dosage, like for instance the Karl Fischermethod, in order to check the amount of remaining water present in theproduct under drying.

The obtaining of rifaximin α or of rifaximin β during drying in factdepends on the final water residual content, higher or lower than 4.5%,and not from the experimental conditions of pressure and temperature atwhich this critical limit of water percent is achieved. The twopolymorphous forms, with higher or lower water content, can be obtainedby drying under vacuum or at atmospheric pressure, at room temperatureor at high temperatures, with or without drying agents, provided thatdrying is prolonged for the time necessary so that the water percentcharacteristic for each polymorphous form is achieved.

Polymorphous form β is obtained when the drying of the productcrystallized and washed with water is stopped at values of water higherthan 4.5%, measured with the Karl Fisher method, preferably between 5.0%and 6.0%, whereas if drying continues to values lower than 4.5%,preferably between 2.0% and 3.0%, polymorphous form α is obtained.

Both form γ and forms α and β of rifaximin are hygroscopic, absorb waterin a reversible way in time in the presence of suitable conditions ofpressure and humidity in the ambient and are susceptible oftransformation from one form to another.

Polymorphous form α, kept in an ambient with a relative humidity higherthan 50% for a period of time between 12 and 48 hours, turns into thepolymorphous form β, which in its turn, by drying until getting anamount of water lower than 4.5%, preferably comprised between 2.0% and3.0%, turns into the polymorphous form α.

Another type of transition takes place between form γ and forms α and β,depending upon the temperatures kept during the phase of precipitationof rifaximin.

In particular form γ turns into forms α or β by keeping a suspension ofform γ of rifaximin in a solvent mixture ethyl alcohol/water 7:3 (V/V)at a temperature between 38° C. and 50° C. under strong stifling for aprolonged period of time, preferably comprised between 6 and 36 hours.

After filtration and washing with demineralized water, drying up to awater content higher than 4.5%, preferably between 5.0% and 6.0%, givesthe polymorphous form β, whereas if drying continues up to a watercontent lower than 4.5%, preferably between 2.0% and 3.0%, form α isobtained.

Rifaximins α and β turn can turn into rifaximin γ by dissolution inethyl alcohol and by treatment of the resulting solution as previouslydescribed for the preparation of form γ.

These transitions from one form to another result to be very importantin the scope of the invention, because they can be an alternativemanufacturing method to obtain the form desired for the production ofthe medicinal preparations. Therefore both the process that allows toturn rifaximin γ into rifaximin α or β in a valid way from an industrialstandpoint, the process that allows to turn rifaximin α or β intorifaximin γ in a valid way from an industrial standpoint, the processthat allow to turn rifaximin α into rifaximin β in a valid way from anindustrial stand point or vice versa rifaximin β into rifaximin α, areimportant part of the invention.

The process concerning the transformation of rifaximin γ into rifaximinα or rifaximin β comprises suspending rifaximin γ in a solvent mixtureconsisting of ethyl alcohol/water in a 7:3 volumetric ratio, heating thesuspension up to a temperature between 38° C. and 50° C. and keeping itat this temperature under strong stifling for a period of time between 6and 36 hours. The suspension is then filtered, the solid is washed withwater and dried getting the polymorphous form β when drying is carriedon until a water percent between 5.0% and 6.0% measured with the KarlFischer method, and polymorphous form α when drying is continued until awater percent between 2.0% and 3.0% is reached.

The process for getting form γ starting from rifaximin α or β comprisesdissolving under stirring, at a temperature between 50° C. and 60° C., αor β form in ethyl alcohol, adding demineralized water until reaching a7:3 ethyl alcohol/water volumetric ratio, cooling the solution understrong stifling to 30° C., cooling the abundant precipitate to 0° C. andkeeping the suspension under stirring at 0° C. for a period of timebetween 6 and 24 hours. The suspension is then filtered, the solid iswashed with water and dried up to a water percent lower than 2.0% thusobtaining rifaximin γ.

The process concerning the transformation of form α into form β consistsin keeping rifaximin α, in the powder form, in an ambient having a rateof relative humidity higher than 50% for the required amount of time,generally between 12 and 48 hours, in order to get a water content inthe powder higher than 4.5%.

The process concerning the transformation of form β into form α consistsin submitting the powder of rifaximin β to a drying process under vacuumor under conditions of normal pressure, with or without a drying agent,at a temperature between the room temperature and 105° C., for a periodof time between 2 and 72 hours, in order to get a water content in thepowder lower than 4.5%, preferably between 2.0% and 3.0%.

From what said above, it results that during the phase of preservationof the product particular care has to be taken so that ambientconditions do not change the water content of the product, by preservingthe product in an ambient having controlled humidity or in closedcontainers that do not allow a significant exchange of water with theexterior ambient.

The polymorph called rifaximin α is characterized by a water contentlower than 4.5%, preferably between 2.0% and 3.0% and from a powderX-ray diffractogram (reported in FIG. 1) which shows peaks at the valuesof the diffraction angles 2θ of 6.6°; 7.4°; 7.9°; 8.8°; 10.5°; 11.1°;11.8°; 12.9°; 17.6°; 18.5°; 19.7°; 21.0°; 21.4°; 22.1°. The polymorphcalled rifaximin β is characterized by a water content higher than 4.5%,preferably between 5.0% and 6.0%, and by a powder X-ray diffractogram(reported in FIG. 2) which shows peaks at the values of the diffractionangles 2θ of 5.4°; 6.4°; 7.0°; 7.8°; 9.0°; 10.4°; 13.1°; 14.4°; 17.1°;17.9°; 18.3°; 20.9°.

The polymorph called rifaximin γ is characterized by a powder X-raydiffractogram much poorer because of the poor crystallinity; thesignificant peaks are at the values of the diffraction angles 2θ of5.0°; 7.1°; 8.4° as reported in FIG. 3.

The diffractograms have been carried out by means of the Philips X'Pertinstrument endowed with Bragg-Brentano geometry and under the followingworking conditions:

X-ray tube: Copper

Radiation used: K (α1), K (α2)

Tension and current of the generator: KV 40, mA 40

Monocromator: Graphite

Step size: 0.02

Time per step: 1.25 seconds

Starting and final angular 2θ value: 3.0°÷30.0°

The evaluation of the water content present in the analyzed samples hasalways been carried out by means of the Karl Fischer method.

Forms α, β and γ can be advantageously used in the production ofmedicinal preparations having antibiotic activity, containing rifaximin,for both oral and topical use. The medicinal preparations for oral usecontain rifaximin α or β or γ together with the usual excipients asdiluting agents like mannitol, lactose and sorbitol; binding agents likestarches, gelatines, sugars, cellulose derivatives, natural gums andpolyvinylpyrrolidone; lubricating agents like talc, stearates,hydrogenated vegetable oils, polyethylenglycol and colloidal silicondioxide; disintegrating agents like starchs, celluloses, alginates, gumsand reticulated polymers; coloring, flavoring and sweetening agents.

All the solid preparations administrable by oral route can be used inthe scope of the present invention, for instance coated and uncoatedtablets, capsules made of soft and hard gelatine, sugar-coated pills,lozenges, wafer sheets, pellets and powders in sealed packets.

The medicinal preparations for topical use contain rifaximin α or β or γtogether with the usual excipients like white petrolatum, white wax,lanoline and derivatives thereof, stearylic alcohol, propylenglycol,sodium lauryl sulfate, ethers of the fatty polyoxyethylene alcohols,esters of the fatty polyoxyethylene acids, sorbitan monostearate,glyceryl monostearate, propylene glycol monostearate, polyethyleneglycols, methylcellulose, hydroxymethylpropylcellulose, sodiumcarboxymethylcellulose, colloidal aluminium and magnesium silicate,sodium alginate.

All topical preparations can be used in the scope of the presentinvention, for instance ointments, pomades, creams, gels and lotions.

The invention is herein below illustrated by some non-limiting examples:from what described it is evident that forms α, β and γ can be obtainedby suitably combining between them the above mentioned conditions ofcrystallization and drying.

Example 1 Preparation of Raw Rifaximin α and of Dried Raw Rifaximin

In a three-necked flask equipped with mechanic stirrer, thermometer andreflux condenser, 120 ml of demineralized water, 96 ml of ethyl alcohol,63.5 g of rifamycin O and 27.2 g of 2-amino-4-methylpyridine are loadedin succession at room temperature. After loading, the mass is heated at47±3° C., is kept under stifling at this temperature for 5 hours, thenis cooled to 20±3° C. and, in 30 minutes, is added with a mixture,prepared separately, consisting of 9 ml of demineralized water, 12.6 mlof ethyl alcohol, 1.68 g of ascorbic acid and 9.28 g of aqueousconcentrated hydrochloric acid. When the addition is over, the mass iskept under stirring for 30 minutes at an inner temperature of 20±3° C.and then, at the same temperature, 7.72 g of concentrated hydrochloricacid are dropped until pH 2.0.

When the addition is over, the mass is kept under stirring, still at aninner temperature of 20° C., for 30 minutes, then the precipitate isfiltered and washed with a mixture consisting of 32 ml of demineralizedwater and 25 ml of ethyl alcohol. The obtained “raw rifaximin” (89.2 g)is dried under vacuum at room temperature for 12 hours yielding 64.4 gof “dried raw rifaximin” having a 5.6% water content and a diffractogramcorresponding to polymorphous form β. The product is further dried undervacuum until constant weight yielding 62.2 g of dried raw rifaximinhaving a 2.2% water content, whose diffractogram corresponds topolymorphous form α.

The product is hygroscopic and the obtained polymorphous form isreversible: polymorphous form α absorbs water from the atmospherichumidity until reaching, depending upon the relative humidity and thetime of exposure, a water content higher than 4.5% and turning intopolymorphous form β which in its turn, by drying loses part of the waterturning into polymorphous form α with a water content between 2.0% and3.0%.

Example 2 Preparation of Rifaximin γ

163 ml of ethyl alcohol and 62.2 g of dried raw rifaximin are loaded atroom temperature into a three-necked flask equipped with mechanicstirrer, thermometer and reflux condenser. The suspension is heated at57±3° C. under stifling until complete dissolution of the solid and atthis temperature 70 ml of demineralized water are added in 30 minutes.When the addition is over the temperature is brought to 30° C. in 40minutes and is kept at this value until abundant crystallization, thenthe temperature is further lowered to 0° C. during 2 hours and kept atthis value for 6 hours. The suspension is then filtered and the solid iswashed with 180 g of demineralized water.

After drying under vacuum at room temperature until constant weight,52.7 g of pure rifaximin γ are obtained with a 1.5% water content.

Form γ is characterized by a powder X-ray diffractogram showingsignificant peaks at diffraction angles 2θ of 5.0°; 7.1°; 8.4°.

Example 3 Preparation of Rifaximin α

62.2 g of dried raw rifaximin and 163 ml of ethyl alcohol are loaded atroom temperature into a three-necked flask equipped with mechanicstirrer, thermometer and reflux condenser. The suspension is heated at57±3° C. until complete dissolution of the solid and then 70 ml ofdemineralized water are added at this temperature during 30 minutes.When addition is over the temperature is taken to 30° C. for 40 minutesand is kept at this value until abundant crystallization. The suspensiontemperature is then taken to about 40° C. and kept at this value during20 hours under stirring; then the temperature is taken to 0° C. in 30minutes and the suspension is immediately filtered. The solid is washedwith 180 ml of demineralized water and dried under vacuum at roomtemperature until constant weight; 51.9 g of rifaximin form α with a2.5% water content and a powder X-ray diffractogram showing peaks atvalues of angles 2θ of 6.6°; 7.4°; 7.9°; 8.8°; 10.5°; 11.1°; 11.8°;12.9°; 17.6°; 18.5°; 19.7°; 21.0°; 21.4°; 22.1°.

Example 4 Preparation of Rifaximin α

89.2 g of raw rifaximin and 170 ml of ethyl alcohol are loaded at roomtemperature into a three-necked flask equipped with mechanic stirrer,thermometer and reflux condenser, then the suspension is heated at 57±3°C. until complete dissolution of the solid. The temperature is taken to50° C. and then 51.7 ml of demineralized water are added at thistemperature in 30 minutes. When addition is over the temperature istaken to 30° C. in one hour and the suspension is kept for 30 minutes atthis temperature obtaining a abundant crystallization. The suspensiontemperature is taken to 40° C. and kept at this value for 20 hours understifling and then further lowered to 0° C. in 30 minutes, after whichthe suspension is immediately filtered. The solid is washed with 240 mlof demineralized water and dried under vacuum at 65° C. until constantweight obtaining 46.7 g of rifaximin α with a 2.5% water content.

Example 5 Preparation of Rifaximin α

Example 3 is repeated by increasing to 50° C. the temperature at whichthe suspension is kept and lowering to 7 hours the time in which thesuspension is kept at this temperature. The obtained product is equal tothat of example 3.

Example 6 Preparation of Rifaximin β

Crystallization of dried raw rifaximin is carried out according to themethod described in example 3. The drying under vacuum at roomtemperature is checked by means of the Karl Fischer method and isstopped when the water content reaches 5.0%:52.6 g of rifaximin β areobtained characterized by a powder X-ray diffractogram showing peaks atvalues of angles 2θ of 5.4°; 6.4°; 7.0°; 7.8°; 9.0°; 10.4°; 13.1°,14.4°; 17.1°; 17.9°; 18.3°; 20.9°.

Example 7 Preparation of Rifaximin α Starting from Rifaximin γ

5 Grams of rifaximin γ are suspended in a mixture consisting of 13 ml ofethyl alcohol and 5.6 ml of water and the suspension is heated at 40° C.for 24 hours under stifling in a 50 ml flask equipped with condenser,thermometer and mechanic stirrer. The suspension is then filtered andthe solid is washed with water and then dried under vacuum at roomtemperature until constant weight. 4 g of rifaximin are obtained showinga powder X-ray diffractogram corresponding to that of the polymorphousform α and a 2.6% water content.

Example 8 Preparation of Rifaximin γ Starting from Rifaximin α

15 Grams of rifaximin form α and 52.4 ml of ethyl alcohol are loadedinto a 250 ml three-necked flask equipped with reflux condenser,thermometer and mechanical stirrer; the suspension is heated understirring at the temperature of 50° C. until complete dissolution of thesolid.

The limpid solution is added with 22.5 ml of water for 30 minutes understirring, cooled to 30° C. and kept at this temperature for 30 minutes.The formed suspension is cooled to 0° C. under strong stifling and keptat this temperature during 6 hours. A part of the suspension is takenafter this period of time, filtered, washed with demineralized water anddried under vacuum at 30° C. until constant weight.

The obtained product, 3.7 g, shows a diffractogram consistent with thatof form γ and α 1.7% water content.

The remaining part of the suspension is kept at 0° C. for further 18hours under strong stirring and then is filtered, washed withdemineralized water and dried at 30° C. under vacuum until constantweight. 9 g of product showing a diffractogram consistent with that ofform γ and a 1.6% water content are obtained.

Example 9 Preparation of Rifaximin α Starting from Rifaximin β

5 Grams of rifaximin β having a 5.0% water content are dried undervacuum at +30° C. for 8 hours obtaining 4.85 g of rifaximin α having a2.3% water content.

Example 10 Preparation of Rifaximin β Starting from Rifaximin α

5 g of rifaximin α having a 2.5% water content are kept during 40 hoursin an atmosphere containing a 56% relative humidity produced by means ofa saturated aqueous solution of calcium nitrate tetrahydrate. 5.17 g ofRifaximin β with a 5.9% water content are obtained after this time.

1. Rifaximin in polymorphic form α.
 2. Rifaximin in polymorphic form β.3. Rifaximin in polymorphic form γ.